This significantly simplifies the dispatch process by creating a
proper backing store for the tiles. Most values that were
calcualted on the fly are now stored as constants in the tile
store.
Also, all tile updated are run through the dirty tile list which
solved a checken-and-egg problem of which order to do sprites vs
new tiles and affords a lot of optimizations since tile rendering
is deferred and each tile is only drawn at most once per frame.
